1. Field of the Invention
The present invention relates to a metal oxide thin film substrate for an organic light-emitting device (OLED) and a method of fabricating the same, and more particularly, to a metal oxide thin film substrate for an OLED which has superior light extraction efficiency and can be easily fabricated at low cost and a method of fabricating the same.
2. Description of Related Art
In general, an organic light-emitting device (OLED) includes an anode, a light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into a hole injection layer and then migrate from the hole injection layer through a hole transport layer to the organic light-emitting layer, and electrons are injected from the cathode into an electron injection layer and then migrate from the electron injection layer through an electron transport layer to the light-emitting layer. Holes and electrons that are injected into the light-emitting layer recombine with each other in the light-emitting layer, thereby generating excitons. When such excitons transit from the excited state to the ground state, light is emitted.
Organic light-emitting displays including an OLED are divided into a passive matrix type and an active matrix type depending on a mechanism that drives an N*M number of pixels which are arranged in the shape of a matrix.
In an active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least two thin-film transistors (TFTs) and one capacitor. Due to this configuration, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform luminance. The active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays.
However, as shown in FIG. 7, only about 20% of light generated by an OLED is emitted to the outside and about 80% of the light is lost by a waveguide effect originating from the difference in the refractive index between a glass substrate 10 and an organic light-emitting layer 30 which includes an anode 20, a hole injection layer, a hole carrier layer, a light-emitting layer, an electro carrier layer and an electron injection layer and by a total internal reflection originating from the difference in the refractive index between the glass substrate 10 and the air. Specifically, the refractive index of the internal organic light-emitting layer 30 ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO) which is generally used for the anode 20 ranges from 1.9 to 2.0. Since the two layers have a very small thickness ranging from 100 to 400 nm and the refractive index of glass used for the glass substrate 10 is about 1.5, a planar waveguide is thereby formed inside the OLED. It is calculated that the ratio of the light that is lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate 10 is about 1.5 and the refractive index of the ambient air is 1.0, when the light is directed outward from the inside of the glass substrate 10, a ray of the light having an angle of incidence greater than a critical angle is totally reflected and is trapped inside the glass substrate 10. Since the ratio of the trapped light is up to about 35%, only about 20% of the generated light is emitted to the outside. Herein, reference numerals 31, 32 and 33 indicate components of the organic light-emitting layer 30. Specifically, 31 indicates the hole injection layer and the hole carrier layer, indicates the light-emitting layer, and 33 indicates the electron injection layer and the electron carrier layer.
In addition, as shown in FIG. 8, in order to overcome the foregoing problem, in the related art, a low index grid (LIP) 50 is formed on the ITO anode 20. The grid 50 converts the direction of the light that travels in the waveguide mode to the front surface, thereby increasing light extraction efficiency.
FIG. 9 shows a simulation result on the OLED shown in FIG. 8. The effect of enhancing the light extraction efficiency is increased when the refractive index of the grid 50 is lower. However, there are problems in that almost no materials have a refractive index of 1.2 or less and that the price of a material is more expensive when the refractive index is lower. In addition, when the grid 50 is formed on the ITO anode 20, as shown in FIG. 8, a stepped portion is formed. Consequently, leakage current may occur. In addition, the OLED shown in FIG. 8 has the problem of difficulty of processing. For example, in some cases, the surface of the anode 20 which adjoins the organic light-emitting layer 30 is metamorphosed in the process of forming the grid 50 on the ITO anode 20, thereby changing the work function. Furthermore, holes are not injected into the organic light-emitting layer 30 through the portion of the anode 20 on which the grid 50 is formed, and the size of the electric field applied thereto is different from the surroundings, thereby decreasing the uniformity of light generated.
In addition, as shown in FIG. 10, in the related art, a convex-concave structure 60 is disposed under the anode 20 (with respect to the paper surface), i.e. in the interface between the anode 20 and the glass substrate 10, in order to enhance light extraction efficiency.
As described above, the anode 20 and the organic light-emitting layer 30 generally act as one light waveguide between the cathode 40 and the glass substrate 10. Accordingly, in the state in which the anode 20 and the organic light-emitting layer 30 act in a waveguide mode, when the convex-concave structure 60 which causes light scattering is formed in the interface adjacent to the anode 20, the waveguide mode is disturbed, so that the quantity of light that is extracted to the outside is increased. However, when the convex-concave structure 60 is formed below the anode 20, the shape of the anode 20 resembles the shape of the convex-concave structure 60 below the anode 20, thereby increasing the possibility that a sharp portion may be localized. Since the OLED has a stacked structure of very thin films, when the anode 20 has a sharp protruding portion, current is concentrated in that portion, which acts as a reason for large leakage current or decreases power efficiency. Accordingly, in order to prevent such deterioration in the electrical characteristics, a flat film 70 is necessarily added when the convex-concave structure 60 is formed below the anode 20. The flat film 70 serves to make the convex and concave portions of the convex-concave structure 60 be flat. When the flat film 70 is not flat and has sharp protruding portions, the anode 20 also has protruding portions, which cause leakage current. Therefore, the flatness of the flat film 70 is very important and thus the maximum surface roughness Rpv is required to be about 30 nm or less.
In addition, the flat film 70 is required to be made of a material, the refractive index of which is similar to that of the anode 20. If the refractive index of the flat film 70 is low, most light is reflected at the interface between the anode 20 and the flat film 70 before being disturbed by the convex-concave structure 60. The light is then trapped between the anode 20 and the organic light-emitting layer 30, which is referred to as the waveguide mode. The flat film 70 is required to be as thin as possible. If the flat film 70 is too thick, more light may be unnecessarily absorbed, and the effect of scattering may be decreased since the distance between the convex-concave structure 60 and the organic light-emitting layer 30 is too large.
However, the process of completely flattening the convex-concave structure 60 using the thin flat film 70 having a thickness of several hundreds of nm is very difficult. In addition, the methods of covering and flattening the convex-concave structure 60 include deposition coating and solution coating. Since the deposition coating is characterized by forming a film following the shape of the convex-concave structure 60, the solution coating is better than the deposition coating when forming the flat film 70. However, at present, it is very difficult to obtain a solution coating material that has a high refractive index, i.e. a refractive index that is equal to or greater than the refractive index of the ITO anode 20, and that satisfies process conditions for polycrystalline thin-film transistors, such as complicated conditions required on the surface of the OLED substrate and high-temperature processing.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.